HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Feb 06, 2013, 12:43 PM
Registered User
FlyBoy20's Avatar
United Kingdom, Wales, Swansea
Joined Aug 2012
696 Posts
Question
Low Kv motor needs SF prop?

Looking at the APC site and it's clones, and there seems to be a bewildering range of prop designs. I've been wondering for months about SF props, and what they are specifically used for? Obviously for slow flying planes like 3Ds (?), so I'm assuming they're for fitting to low-Kv motors? Or else maybe they're for rubber-powered planes? Just dunno...

(Likewise I'm assuming a 'sport' prop is for high-Kv motors...then there is the APC 400 props - most confusing..)

Anyway, I'm looking for a 12x8 for my 910 Kv motor, and to run it with max. efficiency - for a future FunCub. Haven't seen a 12x8 APC type in this size yet, but there is an SF I could get, so should I get it?
FlyBoy20 is offline Find More Posts by FlyBoy20
Reply With Quote
Sign up now
to remove ads between posts
Old Feb 06, 2013, 12:59 PM
Registered User
flydiver's Avatar
United States, WA, Seattle
Joined Jan 2007
5,271 Posts
You are a paddler in a boat, You have a choice of oars, a big fat one, or a skinny one. You can paddle the fat one slowly with high resistance, or the skinnier one fast with less resistance.
They'll both work.
If you try to 'force' the fat one too fast the weak handle will break.

SF props are RPM limited (small hubs, skinny roots-they CAN break), pull more amps, put out more thrust (fat blade). If ALL that is appropriate for your flying use, then use them.

You can get pretty much the same watt draw from a skinnier but larger blade, if that's what you want. After all the mucking about with a wattmeter is done, you use the prop that suits both the motor and how you like the way it flies. I proved to a buddy with dynamic wattmeter tests that a skinny GWS HD prop was the most efficient one he could use for his plane. He tried it and didn't like it so went back to over propping with an APC SF. He couldn't wrap his head around 3/4 throttle with the GWS HD provided the same thrust with LESS amp draw, than 1/2 throttle of the APC SF. You can lead a flier to information.....but.....

BTW-irrespective of the designation props are NOT all the same. For example, GWS SF are floppy and weak, where the APC SF of the same size are stiff. Clone APC props (lots now available) are generally not the quality of APC. I've seen some so poor they were impossible to balance.

Do you have a wattmeter? You need one to have a hope of figuring out prop efficiency.
Plus you don't specify your motor. 910KV is about as descriptive as 8-cylinder. That could be a 40g motor or a 400g motor, which would use entirely different props.
flydiver is offline Find More Posts by flydiver
Last edited by flydiver; Feb 06, 2013 at 03:33 PM.
Reply With Quote
Old Feb 06, 2013, 01:40 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,406 Posts
If you really need a 12 x 8 prop, look for an APC 12 x 8 Thin Electric.
eflightray is offline Find More Posts by eflightray
Reply With Quote
Old Feb 06, 2013, 01:50 PM
Registered User
pda4you's Avatar
USA, TX, Trophy Club
Joined May 2002
14,499 Posts
SF props are used on low power systems. They are best when spun slower and could be appropriate for your cub depending on power. As you can see from the APC site they have a max RPM and you should not exceed that.

I use very few of the SF type props as they are just not very efficient IMHO.

What size plane? How much power will you be using (watts)?

If it is the Multiplex FunCub you need a APCe type prop as recommended above.

Mike
pda4you is offline Find More Posts by pda4you
RCG Plus Member
Reply With Quote
Old Feb 06, 2013, 10:04 PM
Wake up, feel pulse, be happy!
C₄H₁₀'s Avatar
United States, AK, Fairbanks
Joined Aug 2009
12,425 Posts
The idea with a slow flyer prop is to generate a lot of thrust for planes with low-power, lightweight power systems. They're normally used at low RPMs and flight speeds are accordingly slow. Basically, you're using most of the motor's power to generate lower speed and higher thrust than a "normal" prop.

Think of it like the gearing on a bicycle: You might have a 21-speed mountain bike. Three chainrings (30, 36, 44) and seven cogs (14, 16, 18, 22, 24, 26, 36). The six lower cogs are your "normal" riding gears - pavement, grass, small hills, big hills, whatever. Those are like normal props. You can get a very wide range of thrust and speed for a given power input.... But then you have that 36-tooth cog that you only use when you're going straight up a cliff. You might be putting a ton of power into it, but only a tiny bit of speed is coming out since the bulk of it is going toward pushing you forward... Ok, well, it made more sense in my head.

The motor's Kv value is a secondary concern. It's really a disservice to yourself to think of a motor's RPMs or power in terms of Kv alone since it doesn't really tell you much at all. I could use a slow-flyer prop on a motor with a Kv value of 500 or 3500 depending on the motor's size and the input voltage.
C₄H₁₀ is offline Find More Posts by C₄H₁₀
Reply With Quote
Old Feb 08, 2013, 11:36 AM
Registered User
Joined Apr 2007
1,961 Posts
Anyone know the max Rpm of a GWS slow fly prop?

Also....are they so floppy and weak that its best to avoid?

I bought a couple different sizes but dont have to use them if they suck.
wopachop is offline Find More Posts by wopachop
Reply With Quote
Old Feb 08, 2013, 11:37 AM
Registered User
pda4you's Avatar
USA, TX, Trophy Club
Joined May 2002
14,499 Posts
They stink. I would recommend the GWS HD direct drive props - they rock.
pda4you is offline Find More Posts by pda4you
RCG Plus Member
Last edited by pda4you; Feb 08, 2013 at 12:30 PM.
Reply With Quote
Old Feb 08, 2013, 12:28 PM
Registered User
flydiver's Avatar
United States, WA, Seattle
Joined Jan 2007
5,271 Posts
Quote:
Originally Posted by wopachop View Post
Anyone know the max Rpm of a GWS slow fly prop?

Also....are they so floppy and weak that its best to avoid?
They are OK for a Slow Stick and low RPM, low output motors.
GWS does not publish RPM limits. If they did likely no one would ever buy the SF again. As I said above "GWS SF are floppy and weak".
The HD's I like a lot.

If you try them and over rev you can hear the prop blades distorting as they get pushed too far. They make kind of a flock of birds taking off sound.
flydiver is offline Find More Posts by flydiver
Reply With Quote
Old Feb 10, 2013, 06:54 AM
Registered User
Joined Jan 2009
7,643 Posts
I use 8" and 9" APC slo-fly props on a lot of models in the one-pound range, eg. GWS parkflyers, a light 48" Cub, 30" balsa 3D planes, and 40" EPP foamies. I use a 10" APC Slo-fly on a 2 pound balsa Katana clone.

Not really appropriate for anything larger than that.
rafe_b is offline Find More Posts by rafe_b
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Question Higher KV & Smaller Prop VS. Lower KV and Bigger Prop fnffishcore Power Systems 4 Jan 22, 2013 10:14 PM
Discussion Low kv motors vrs high kv motors in windy conditions crcr Multirotor Talk 6 Jan 10, 2013 01:34 PM
Discussion low-S high-KV vs high-S low-KV battery-motor set up jy0933 Multirotor Electronics 33 Aug 19, 2012 02:28 AM
Sold Low Time Champ Brick, Motor, and Extra Prop (Motor is a higher kv Mustang motor) Kevin Greene Aircraft - Electric - Micro & Indoor Airplanes (FS/W) 3 Jan 26, 2012 11:37 AM
Discussion low kv bigger prop vs higher kv smaller prop Twisted Realm 3D Flying 3 Jul 13, 2009 11:01 AM